text: Passive devices by definition require no energy. Passive techniques include turbulators or roughness elements geometric shaping, the use of vortex generators, and the placement of longitudinal grooves or riblets on airfoil surfaces. 
  Active control requires actuators that require energy and may operate in a time-dependent manner. Active flow control includes steady or unsteady suction or blowing, the use of synthetic jets, valves and plasma actuators. Actuation may be pre-determined (open-loop control) or be dependent on monitoring sensors (closed-loop control).